Lightweight nonwoven fabric of increased opacity

ABSTRACT

A lightweight nonwoven fabric having a high degree of opacity comprises a nonwoven web of continuous synthetic polyamide filaments autogenously bonded together and short lengths of polyamide ligaments dispersed among the continuous filaments.

United States Patent limerick J. Dobo Cary, N.C.

July 22, 1969 Dec. 28, 1971 Monsanto Company St. Louis, Mo.

inventor Appl. No. Filed Patented Assignee LIGHTWEIGHT NONWOVEN FABRIC OF INCREASED OPACITY 2 Claims, 1 Drawing Fig.

US. 161/ 150, 156/306,161/156, 161/157, 161/170 Int. Cl D0411 3/14, D04h 5/00 FieldoiSearch 161/150, 156, 157, 170

[56] Reierences Cited UNITED STATES PATENTS 3,341,394 9/1967 Kinney 161/150 X 3,501,368 3/1970 Schabert et al. 161/112 3,516,900 6/ 1970 Mallonee et a]. 161/ 150 Primary Examiner-Robert F. Burnett Assistant Examiner-Raymond O. Linker, Jr.

Attorneys-J. Bowen Ross, Jr., R. E. Weinkauf, John D.

Upham and Neal E. Willis ABSTRACT: A lightweight nonwoven fabric having a high degree of opacity comprises a nonwoven web of continuous synthetic polyamide filaments autogenously bonded together and short lengths of polyamide ligaments dispersed among the continuous filaments.

PATENIEUnzczmsm {630, 1

INVENTOR. EMERICK J. DOBO ATTORNEY LIGHTWEIGHT NONWOVEN FABRIC OF INCREASED OPACITY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a nonwoven web and, more specifically, to a spunbonded web weighing less than 1 oz./sq.yd. which is comprised of continuous nylon filaments being autogenously bonded together at a substantial number of filament crossover points.

2. Description of the Prior Art Bonded nonwoven fabrics are old in the art; however, the concept of manufacturing nonwoven webs from continuous synthetic filaments is relatively new. It was not until the 1960's that processes were developed which were capable of attenuating freshly extruded filaments and laying them down onto a moving belt to form a nonwoven web [U.S. Pat. Nos.

3,34l,394, 3,328,992 and 3,276,944]. These patents dealt primarily with polyester and polyolefinic filaments although the use of nylon filaments was casually mentioned. Bonding was accomplished by heat, solvents or adhesives; however, nylon webs are not readily bondable by these techniques. For example, while nylon is a thermoplastic, heating nylon filaments to the melting or softening point destroys substantially all molecular orientation and thus weakens the filaments to the point of uselessness. Solvent bonding of nylon filaments is not practical since the solvating rate is uncontrollable and the use of adhesive binders renders the resulting fabric too boardy and too weak to be of any practical value.

In producing lightweight webs, the use of adhesive binders is further not practical because the weight of the adhesive is generally about 40 percent of the total weight of the fabric. Therefore, the adhesive replaces an equivalent weight of filaments which results in a web having too few filaments to provide adequate cover'or strength. Where the web is bonded either by solvents or by adhesives, the web must be subjected to a liquid or water treatment which, for webs of such light weights, destroys the web geometry since the web does not have the integrity to withstand the liquid prior to bonding.

The lightweight nonwoven webs, which are defined as webs weighing less than 1 oz./sq. yd., currently on the market are produced either by depositing olefinic fibrous materials on a belt with the bonding being carried out by a heat process or by means of depositing submicron organic fibers onto a collection surface as set forth in U.S. Department of Commerce, Office of Technical Services, Publication No. PB 1 l I437. While nonwoven fabrics can be made from nylon in accordance with the publication, the fibers are not continuous and the resulting web has little strength.

SUMMARY OF THE INVENTION In its broadest aspects, the present invention comprehends the formation of a nonwoven web weighing less than 1 oz./sq.yd. and being comprised of continuous nylon filaments preferably of less than denier per filament with the filaments being autogenously bonded together at a substantial number of filament crossover points. The filaments comprising the web may be arranged in an orderless or haphazard manner, that is, without any preferred orientation or they may be directionalized depending upon the ultimate end use of the web.

Autogenous bonding of nylon filaments can be accomplished in accordance with the method set forth in U.S. Pat. No. 3,516,900 filed June 17, 1968 by William C. Mallonee et al. By autogenous bonding, it is meant that bonds are formed between touching filaments without visible polymer migration at the bond points and in the absence of external bonding agent. As disclosed in detail in the above identified application, gaseous hydrogen chloride or like activating gases contact and is absorbed by the nylon filaments at least along the surfaces thereof. The hydrogen chloride molecule in the nylon causes the interchain hydrogen bonds-which exists between adjacent amide groups to break and upon removal or'desorplightweight nonwoven nylon webs of this invention, the

general procedure is set forth in U.S. Pat. No. 3,542,615 by E. .I. Dobo et al. However, the lightweight webs must be handled more delicately than the webs of the mentioned application or the prior art. The webs of this invention are provided with the necessary integrity to enable removal from the laydown belt without being destroyed either by contacting the filaments with hydrogen chloride gas prior to their deposition so that the filaments will be slightly bonded together while on the belt or by spraying a solution of nylon referred to as wet ligaments on the web after its formation on the belt. The nylon filaments may be exposed to the activating gas prior to being deposited on the belt by mixing the activating gas with the pressurized air supplied to the aspirator, the aspirator functioning to atten uate the freshly extruded filaments and to forward them to the collection belt. The activating gas is absorbed by the nylon filaments and upon reaching the belt and while remaining thereon, the hydrogen chloride partially evaporates to form bonds between a substantial number of touching filaments. Similarly, upon the deposition of the wet ligaments in the form of a solution upon the web, the solvent is partially evaporated so that ligaments are formed which bind the continuous nylon filaments together to form a web having strength sufficient to allow it to be withdrawn from the belt without destroying the web geometry.

Therefore, an object of this invention is to provide a nonwoven bonded nylon web weighing less than 1 oz./sq.yd.

Another object of this invention is to provide a nonwoven web weighing less than l oz./sq.yd. which is comprised of continuous nylon filaments having a small percentage by weight of short length nylon fibers.

Another object of this invention is to provide a process for forming a nonwoven web of continuous nylon filaments weighing less than 1 oz./sq.yd. by exposing the filaments to an activating gas prior to the deposition of filaments on a moving belt.

A still further object of this invention is to provide a process for forming a lightweight web of continuous nylon filaments by providing the web with short length nylon ligaments prior to bonding of the continuous filaments.

Other objects and advantages of the present invention and the means of attainment will be apparent from the following description and accompanying drawing.

DESCRIPTION OF THE DRAWING The FIGURE is a schematic diagram of the apparatus used for making the lightweight bonded nonwoven web of this invention and for carrying out the processes of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to the FIGURE, an arrangement is shown for converting polymer to a fabric in a single, continuous process. Reference numeral 10 denotes a conventional melt extruder and associated equipment which is employed to form continuous filaments 12 from a molten thermoplastic material which for the purposes of this invention is a polyamide such as nylon 6, nylon 66, nylon 10 and the like. Extruded filaments 12 are forwarded to and are drawn through aspirator 14 which is pneumatically operated and which is mechanically traversed by means of traversing mechanism 16. It is also contemplated that where a plurality of aspirators are employed, they may be stationarily positioned in a given arrangement so that the filaments forwarded by them to the belt form overlapping patterns to provide a web of substantial uniformity throughout. Motor 18 of a conventional type reciprocates traversing mechanism 16 on a pair of fixed guide bards 20. The aspirator 14 is traversed at a predetermined rate to deposit the filaments 12 in a haphazard or orderless manner on the collection belt 22. As the filaments are deposited upon collection belt 22, they are intermingled to the extent that a coherent web structure is formed.

To overcome the problem of having a nonuniform web selvage, edge forming deflector plates 24 are positioned in close relationship to collection belt 22. These plates are preferably inclined at about 15 from the vertical and are spaced apart at the desired web width. The traverse stroke imparted to the filaments by traverse mechanism 16 is somewhat greater than the ultimate width of the web which results in the selvage having a uniform density.

A suction box 26 is provided in close proximity underneath collection belt 22 for exhausting the air emanating from aspirator l4 and the vacuum created in suction box 26 is advantageously employed to assist in the deposition of the filaments on collection belt 22. Additionally, the negative pressure created in suction box 26 functions to press adjacent filaments together to facilitate bonding when the activating gas is supplied to the aspirator and to facilitate the entwinement of the wet ligaments about the continuous filaments when the nylon solution is sprayed onto the web.

Upon leaving collection belt 22, the web 30 is passed through a pair of pressure rolls 32 to improve web density and coherency prior to entering chamber 34 which is filled with an activated gas introduced at inlet 36 with the excess thereof being exhausted at outlet 38. Some of the activating gases found to be effective are hydrogen chloride, hydrogen bromide, hydrogen floride, borontrifloride, chlorine, and a mixture of chlorine and sulfur dioxide. The resident time in chamber 34 is dependent upon the type of activating gas used and the concentration of the same. After the filaments are sufficiently exposed to the gas to permit surface adsorption thereof, web 30 is passed through press rolls 40 and through wash bath 42 which if filled with water or a mild alkaline solution.

A pair of separately spaced-apart guide members 44 control the horizontal path of web 30 through the bath to permit the desorption of the gas from the filaments whereby bonding occurs from the recrystallization of the intermolecular hydrogen bonds. Thereafter, the self-bonded web is passed over bar 46 and is forwarded to a dryer 47 which is of the conventional type. The web exiting dryer 47 is in condition to be packaged by a conventional means (not shown) such as a takeup roll.

To produce the lightweight webs of this invention, that is, webs weighing less than 1 oz./sq.yd., the web prior to being removed from belt 22 must be provided with sufficient integrity to enable it to withstand the shear and tensile forces exerted upon it during separation from the belt since the unmodified weight webs, as formed, do not have the required tensile strengths or interfilament cohesion. This invention contemplates increasing web tenacity in two ways, which are: slightly bonding the filaments while on belt 22 and supplying the web while on belt 22 with an external binder which is compatible with the reactant gases.

The nylon filaments may be bonded together at their crossover points while remaining on collection belt 22 by supplying the activating gas to the aspirator 14. Thus, aspirator 14 receives pressurized air through line 50 and the activating gas, which may be hydrogen chloride, through line 51. The air and the activating gas are mixed together in aspirator 14 and exit a common orifice. The nylon filaments 12 at least partially absorb the activating gas while in aspirator 14 and are in a bondable condition upon being laid on belt 22. Partial desorption of the activating gas occurs while the filaments remain on belt 22 by the activated gas evaporating under normal operating temperatures to allow at least partial bonding to occur between touching filaments. Where the ratio of pounds of ac tivating gas to pounds of nylon filaments forwarded through the aspirator is high, i.e. about I to l, chamber 34 may be removed from the system since the nylon filaments 12 have absorbed sufficjent quantities of the activating gas to allow complete bonding in a subsequent desorption process which may be washing or by heat or both. However, since only a small portion of the activating gas which enters the system through aspirator 14 can be recovered by means of vacuum box 26, for most purposes, it is economical to have a two stage bonding operation as shown in the FIGURE which would include chamber 34.

The other embodiment for increasing web integrity is by the use of wet ligaments. The ligaments, upon removal of the solvent, are in the form of relatively short fibers of nylon which are adapted to bind the continuous nylon filaments together sufficiently to allow the web to be removed from the collection belt. A convenient and easily manageable liquid solution is comprised of 70 percent formic acid and 30 percent nylon by weight which is supplied to reservoir 53 by means of conduit 54. The solution is under pressure while in reservoir 53 and is directioned onto web 30 by means of nozzle 55. Nozzle 55 may be adapted to oscillate or it may be stationary having a diverging exit such as to distribute the solution over the surface of the web. The use of ligaments while aiding in the removal of the web from the belt also is extremely adaptable to increase web cover or opacity. For example, the ligaments comprise only about 2 percent by weight of the web but increase opacity by at least 50 percent. Sufficient quantities of the formic acid will evaporate while the web remains on belt 22 prior to removal to provide the necessary web integrity for removal from the belt. The remainder of the formic acid or solvent is removed when passing between pressure rolls 32 which for this embodiment are also heated.

The nonwoven web which is comprised of continuous nylon filaments and nylon ligaments is very versatile For example, where fiber surface area is of importance as for filters, the ligaments may comprise as much as 98 percent by weight of the web with the continuous nylon filaments serving as a reinforcing medium. Such webs have excellent drapeability and are adaptable for being molded into intricate shapes.

Where the lightweight webs are in the range of 0.2 oz./sq.yd. or less, it may be useful or even necessary to supply the activating gas to aspirator l4 and to deposit ligaments onto the web. Under such circumstances, the use of gas chamber 34 may or may not be necessary depending upon the amount of activating gas supplied to aspirator 14 in relation to the quantity of filaments passing therethrough. Thus, extremely lightweight webs can be made in accordance with this invention with the webs having a high degree of opacity.

lclaim:

1. A lightweight nonwoven fabric having increased opacity comprised of continuous synthetic polyamide filaments arranged in the form of a nonwoven web, said web having a predetermined opacity, and short lengths of polyamide ligaments being intermingled with said continuous filaments and being evenly dispersed across said web, said ligaments comprising about 2 percent of said fabric by weight and increasing the predetermined opacity of said web by at least 50 percent, said continuous filaments being autogenously bonded together without visible polymer migration at a substantial number of filament crossover points.

2. The fabric of claim 1 wherein said fabric weighs less than 1.0 oz./yd.

* i k t t 

2. The fabric of claim 1 wherein said fabric weighs less than 1.0 oz./yd.2. 